Conventionally, an ultrasound diagnosis apparatus employs a method of focusing a transmission beam and a reception beam of ultrasound waves in order to increase a lateral resolution of an ultrasound image. Particularly, an ultrasound diagnosis apparatus using an ultrasound probe including an array vibrator of an electronic scan type uses an electronic focusing method of adding reception signals of respective channels by delay-time control.
However, according to the electronic focusing method, a reception beam diffuses at a deep location distant from a focus point, consequently, a lateral resolution of an ultrasound image decreases. For this reason, a general ultrasound diagnosis apparatus uses a dynamic focusing method. The dynamic focusing method is a method of performing delay-time control such that a focus point continuously moves in a depth direction with time when receiving a reception beam, accordingly, a reception beam can be constantly obtained from a region a beam is focused.
The delay-time control according to the dynamic focusing method is explained below with reference to FIG. 13. FIG. 13 is a schematic diagram for explaining the delay-time control according to the dynamic focusing method.
The following description explains each position of an ultrasound-wave scanning plane by using a coordinate system having the origin at the center of an aperture of a vibrator present in a receiving aperture of an ultrasound probe, and including “a coordinate in a depth direction from the ultrasound probe”, and “a coordinate in the traverse direction from the center of the aperture”.
As shown in FIG. 13, when a focus point subjected to the delay-time control is a point positioned at coordinates (X, 0), a difference (delay time: Δt) between a time until a wavefront of a reflected sound wave generated at the focus point P reaches a vibrator positioned at coordinates (0, Y), and a time until the wavefront of the reflected sound wave generated at the focus point P reaches a vibrator positioned at the origin (0, 0) is defined by a difference (t) between distances of the focus point P and each vibrator. Such time difference is expressed by Expression (1) below. Where “C” described in Expression (1) denotes a sound velocity inside a medium subjected to ultrasound-wave scanning.
                              Δ          ⁢                                          ⁢          t                =                                                                              X                  2                                +                                  Y                  2                                                      -            X                    C                                    (        1        )            
According to the dynamic focus method, a delay time at each vibrator is calculated by using Expression (1), with respect to each of different focus points. Consequently, according to the dynamic focusing method, a delay time (reception delay time) when adding a signal (reception signal) of a reception beam received by each vibrator of an ultrasound probe is determined. Consequently, according to the dynamic focusing method, a distribution (delay distribution) that reception delay times of respective vibrators are determined is set at each focus point of the ultrasound-wave scanning plane. The ultrasound diagnosis apparatus that executes the dynamic focusing method focuses reception signals from respective focus points in different depth directions by adding them by using a reception delay time obtained from the delay distribution, thereby improving the lateral resolution of an ultrasound image.
The ultrasound diagnosis apparatus that executes the dynamic focusing method generally sets a delay distribution by assuming that a sound velocity “C” is a typical sound velocity of a diagnosis portion to be imaged. However, there is a report that sound velocity values inside a living body are different in portions (for example, “muscle: 1560 m/sec”, and “fat: 1480 m/sec”). Moreover, there is a report that sound velocity values inside a living are different between subjects even in the same portion.
Therefore, when there is a difference between a sound velocity that is set (hereinafter, referred to as set sound velocity) and a sound velocity of a living body in a diagnosis portion (hereinafter, referred to as living-body sound velocity), a difference is produced between “a reception delay time for actually focusing a reception signal at a focus point” and “a reception delay time calculated by using Expression (1)”. In other words, when there is a difference between a set sound velocity and a living-body sound velocity, a focus point for focusing a reception signal deviates, consequently, even by using the dynamic focusing method, the lateral resolution decreases and the image quality of an ultrasound image decreases.
For example, as shown in the left side of FIG. 14A, when the set sound velocity and the living-body sound velocity are equal, a focus point does not deviate at each position in different depth direction (Fn−1, Fn, Fn+1), so that the lateral resolution in each depth direction improves. For example, as shown in the right side of FIG. 14A, on an ultrasound image onto which a phantom is imaged, signals originating phantom are rendered as a point without blur.
However, as shown in the left side of FIG. 14B, for example, when a set sound velocity is smaller than a living-body sound velocity, a focus point deviates at each position in different depth direction (Fn−1, Fn, Fn+1), so that the lateral resolution in each depth direction decreases. For example, as shown in the right side of FIG. 14B, on an ultrasound image onto which a phantom is imaged, signals originating phantom blur in the traverse direction, so that the lateral resolution of the ultrasound image decreases. FIGS. 14A and 14B are schematic diagrams for explaining decrease in lateral resolution caused by a difference between a set sound velocity and a living-body sound velocity.
For this reason, in order to detect a living-body sound velocity in a diagnosis portion, technologies of such as a reflection method and a phase correction according to a cross-correlation method are known. However, according to such technologies, there are constraints that a reflective body, such as a calculus or a boundary wall, needs to be present in a detection area of a living-body sound velocity, and furthermore, a reflective body has to be a point, consequently, such technologies cannot be generally used.
Therefore, in order to calculate a reception delay time that guarantees the lateral resolution of an ultrasound image, a technology of optimizing a set sound velocity has been recently known (for example, see JP-A 2008-264531 (KOKAI)). Specifically, an ultrasound diagnosis apparatus creates a plurality of ultrasound images by using respective reception delay times that are calculated with different sound velocities. The ultrasound diagnosis apparatus then divides each ultrasound image into a plurality of small areas, and calculates a contrast value (for example, a variance of amplitude values) with respect to each of the small areas.
The ultrasound diagnosis apparatus then determines that an optimal sound velocity in each small area is a set sound velocity with which a contrast value is at the maximum, and uses the determined optimal sound velocity in each small area as the sound velocity (set sound velocity) when calculating a reception delay time in the corresponding small area. In other words, by determining an optimal sound velocity in each small area, the lateral resolution of an ultrasound image can be guaranteed.
However, according to the conventional technologies described above, although an optimal sound velocity in a particular area is determined with objective numerical values (contrast values); when the calculation precision of the contrast values is insufficient, there is a case where the lateral resolution of a portion to be focused by an engineer who takes an ultrasound image or an image reader who reads an ultrasound image does not always become optimal, even if the ultrasound image is created by the determined optimal sound velocity.